The present invention generally relates to antennas, and relates in particular to antenna systems that include one or more monopole antennas.
Monopole antennas typically include a single pole that may include additional elements with the pole, including for example, additional monopole antennas. Non-monopole antennas generally include antenna structures that form two or three dimensional shapes such as diamonds, squares, circles etc.
As wireless communication systems (such as wireless telephones and wireless networks) become more ubiquitous, the need for smaller and more efficient antennas such as monopole antennas (both large and small) increases. Many monopole antennas operate at very low efficiency yet provide satisfactory results. In order to meet the demand for smaller and more efficient antennas, the efficiency of such antennas must improve.
Further, the adjustment or tuning of the operating frequency of an antenna is sometimes required. Such tuning, however, is typically available only over a small range. Adjustment of an antenna over a wide operating frequency range of, for example, up to 2:1 or more generally requires a number of antennas or requires base-loading (sometimes called base-tuning). Base-loading involves matching the antenna load presented to the transmitter by varying the antenna load. The efficiency of such systems, however, is generally low and radiation performance of such antennas will vary widely over the full tuning range of the antenna. Efficiency or antenna gain can vary widely from one end of this tuning range to the other.
For example, base-loaded antennas may have efficiency or gain from a high of 60% to a low of less than 10%. The lower gain is usually associated with the lowest frequency. An antenna with an efficiency or gain of 10% will radiate 1 Watt out of every 10 the transmitter loads into the tuner. This generally results in very robust tuner designs when high power is utilized. A 5 KW transmitter at an impedance of 50 Ohms will be capable of supplying 10 amps of average RF current operating in the continuous mode. This may range to peaks as high as 15 amps or more when amplitude modulation is used. If these 10 to 15 amps of RF current are transformed from 50 Ohms to an impedance that is much higher, then the tuner must be designed to withstand extremely either high voltages or high currents. Either way, it becomes a significant problem at higher power levels to control the antenna matching and maintain efficiency.
As mentioned above, a number of antennas may be used instead of the base-loading technique to achieve wide bandwidth operation. Such a multi-antenna system may include an antenna for each desired frequency. Each antenna may be designed to present a constant 50 Ohm load at the operating frequency confined within some bandwidth. Another alternative involves lengthening and shortening a common antenna by inserting and removing sections of tubing as needed or using a telescoping mast antenna. Telescoping mast antennas present problems in achieving the lowest and highest frequency of operation as the necessary steps for adjusting the antenna are time consuming and labor intensive. For example, for a ¼ wave monopole antenna this typically requires that the antenna be taken apart and re-assembled using longer sections
There is a need, therefore, for more efficient and cost effective implementation of a monopole antenna, as well as other types of antennas and antenna systems, and there is a further need for an efficient and cost effective method for tuning such antenna systems. For example, there is a need in particular for a method of rapidly changing the antenna resonance to any desired frequency within its range and while maintaining a constant bandwidth to provide a constant 50 Ω, match to the transmission line connected to the transmitter or final amplifier. The mechanism for accomplishing this must have the capability of handling the large radio frequency current and transforming this into radiation by the antenna. It is desirable, for example, to provide an antenna designed for typical operation within the AM broadcast band of 535-1700 kHz, and to have a 30 kHz bandwidth (+/−15 kHz).